The present invention relates to a modified cyanate ester group curable resin composition, and a varnish, prepreg, metal clad laminated board, film, printed circuit board, and multilayered circuit board using the same.
A large volume of data is required to be processed with a high speed in a highly information-oriented society, and consequently the, frequency of signals used in computers and information terminals has become higher and higher in recent years. However, since an electric signal has a property that its transmission loss becomes larger as the frequency becomes higher, developing a low-loss printed circuit board capable of coping with such high frequencies has become an important requirement of the industry.
The transmission loss of a printed circuit board comprises a conductor loss depending on the shape of the circuit (conductor), a skin-effect resistance, a characteristic impedance, and the like, as well as a dielectric loss depending on the dielectric properties of the insulating layer (dielectric) around the circuit. The dielectric loss is dominant in the overall electric loss in a high frequency circuit. Therefore, in order to reduce the transmission loss of the high frequency circuit, it is apparent that reducing the dielectric constant and the dissipation factor (tan xcex4) of the printed circuit board (particularly, the insulating resin) is necessary. For example, in the field of mobile communication equipment dealing with high frequency signals, printed boards having a low dissipation factor are strongly required in order to reduce the transmission loss in a quasi-microwave band (1 to 3 GHz) as the frequency of signals is increased.
On the other hand, in the field of electronic information equipment, such as computers the, development of high speed microprocessors having an operating frequency exceeding 200 MHz and an increase of signal frequency has been advancing rapidly in order to handle increasinly larger volumes of information in a short time. In the equipment using such high speed pulse signals the, signal delay time on the printed circuit board becomes a problem. Since the signal delay time on the printed circuit board becomes longer in proportion to the square root of a specific dielectric constant xcex5 r of the insulator around the circuit, resins having a low dielectric constant are required for circuit boards used in a computer and the like.
Regarding resin compositions for improving the high frequency property of the printed circuit board and which are capable of coping with the trend toward the use of increasingly higher frequency signals, as described above, a method using a Cyanate ester/epoxy resin composition has been disclosed in JP-B-46-41112 (1971), and a method using a bismaleimide/Cyanate ester/epoxy resin composition has been disclosed in JP B-52-31279 (1977), as a composition using a Cyanate ester resin having the lowest dielectric constant among thermosetting resins.
As a method of improving the high frequency property using a thermoplastic resin, methods of using polyphenylene ether group resin compositions having a desirable dielectric property among heat-resistant thermoplastic resins, such as a resin composition composed of a polyphenylene ether resin (PPO or PPE) and a cross linking polymer/monomer has been disclosed in JP-B-5-77705 (1993), and a resin composition composed of a polyphenylene ether having a specific curable functional group and a cross linking monomer has been disclosed in JP-B-6-92533 (1994).
Further, as a means of improving the high frequency property using a resin composition composed of a cyanate ester resin having a low dielectric constant and a polyphenylene ether resin having a desirable dielectric property, a method of using it a resin composition composed of a cyanate ester/bismaleimide and a polyphenylene ether resin has been disclosed in JP-B-63-33506 (1988), and a method of using a resin composition composed of a phenol modified resin/cyanate ester reactant and polyphenylene ether resin has been disclosed in JP-A-5-311071 (1993). Furthermore, a resin composition prepared by kneading a polyphenylene ether resin and a cyanate ester resin has been disclosed in JP-B-61-18937 (1986) as a heat resistant molding material having a desirable frequency characteristics.
The methods disclosed in JP-B-46-41112 (1971) and JP-B-52-31279 (1977), respectively, had a problem in that the high frequency property was insufficient though the dielectric constants were slightly lowered, because the resin compositions contained thermosetting resins other than a cyanate ester resin.
The methods disclosed in JP-B-5-77705 (1993) and JP-B-6-92533 (1994) also had a problem in that the resin compositions were high in molten viscosity and lacked in resin flow, though their dielectric constants were somewhat improved, because the main component of the resin compositions was a polyphenylene ether resin which was essentially a thermoplastic resin. Therefore, the resin compositions required a high temperature and a high pressure for pressurized molding of the laminated board, and were unsuitable for forming a multilayer printed circuit board, which is required to fill a very small space in a circuit pattern, because of insufficient moldability.
The methods disclosed in JP-B-63-33506 (1988)and JP-A-5-311071 (1993) had a problem in that the high frequency property was still insufficient, though the dielectric constants were somewhat improved, because the thermosetting resin used together with the polyphenylene ether resin was a bismaleimide/cyanate ester resin or a phenol modified resin/cyanate ester reactant. When the quantity of polyphenylene ether resin was increased, the resin composition became high in molten viscosity and lacked in fluidity, thereby to decrease the moldability, similar to the case of the polyphenylene group resin composition described above.
The resin composition prepared by kneading polyphenylene ether resin and cyanate ester resin disclosed in JP-B-61-18937 (1986) had a desirable dielectric property and a relatively preferable moldability, because the molten viscosity was-lowered by being modified by cyanate ester resin. However when cyanate ester was singly used as a curing composition, the problem that the dielectric property of the cured resin had a high dissipation factor, while the dielectric constant was relatively low, was still remained. Further, when the quantity of cyanate ester was decreased (an adding quantity of polyphenylene ether resin was increased) in order to lower the dissipation factor, the problem that the molten viscosity of the resin composition was increased to make the fluidity insufficient still remained, and the moldability was decreased, as similar to the case of the polyphenylene group resin composition described above.
A first object of the present invention is to provide a modified cyanate ester group curable resin composition having a desirable heat resistance, a similar moldability and workability as a conventional thermosetting resin, such as an epoxy resin, a low dielectric property, and a low dissipation factor and a low loss property in the high frequency band.
A second object of the present invention is to provide a flame retardant resin film using a modified cyanate ester group curable resin composition, and a method for manufacturing the same.
An overall object of the present invention is to provide a resin film using a modified cyanate ester group curable resin composition, and a method for manufacturing the same.
A third object of the present invention is to provide a modified cyanate ester group curable resin varnish for a printed circuit board using a modified cyanate ester group curable resin composition, and a method for manufacturing the same.
A fourth object of the present invention is to provide methods of manufacturing prepreg for a laminated board and metal clad laminated board, using a modified cyanate ester group curable resin varnish.
A fifth object of the present invention is to provide a multilayered circuit board manufactured by laminating a printed circuit board and/or an inner layer circuit board, which are manufactured by patterning the circuit on the laminated board manufactured by the above method of manufacturing prepreg for a laminated board and a metal clad laminated board, and for a prepreg, film, and/or a film adhered with metal foil, which are manufactured the above method, and forming circuits for making connections between inner circuits to each other and to the metal foil.
The first object of the present invention can be achieved with a modified cyanate ester group curable resin composition comprising essentially (A) a cyanate ester group compound expressed by the chemical formula (1), (B) a monovalent phenolic group compound expressed by any one of the chemical formula (2) and (3), (C) a polyphenylene ether resin, (D) a flame retardant not reactive with the cyanate ester group compound and (E) a metal group reaction catalyst. 
where, R1 is 
and respective of R2 and R3 is hydrogen or a methyl group, and both can be the same or different from each other) 
(where, respective of R4 and R5 is any one of a hydrogen atom and low alkyl group having carbon number of 1 to 4, and both can be the same or different from each other. n is a positive integer of 1 or 2.) 
(where, R6 is 
or 
It is preferable to use a modified cyanate ester group curable resin composition, which is prepared by mixing (B) the monovalent phenolic group compound expressed by the chemical formula (2) or (3) at 4 to 30 parts by weight with 100 parts by weight of (A) the cyanate ester group compound expressed by the chemical formula (1).
The second object of the present invention can be achieved by a flame-retardant resin film manufactured by semi-curing or curing a modified cyanate ester group curable resin composition including the flame-retardant not reactive with the cyanate ester group compound. The method of manufacturing the flame-retardant resin film can be achieved by a method comprising the steps of applying the varnish comprising the modified cyanate ester group curable resin composition including the flame retardant and a solvent onto one side plane of a carrier by a flowing method, and removing the solvent by heating and drying to form the film.
The present invention can be also achieved by the modified cyanate ester group curable resin film manufactured by semi-curing or curing the modified cyanate ester group curable resin composition, which does not include any flame-retardant not reactive with the cyanate ester group compound. The method of manufacturing the resin film can be achieved by a method
The third object of the present invention can be achieved with the modified cyanate ester group curable resin varnish comprising essentially (F) an aromatic hydrocarbon group solvent, and (G) a ketone group solvent, in addition to the modified cyanate ester group curable resin composition. The method of manufacturing the varnish with the modified cyanate ester group curable resin composition for the printed circuit board can be achieved by a method including the steps of dissolving (C) polyphenylene ether resin into (F) the aromatic hydrocarbon group solvent by heating, reacting subsequently (A) the cyanate ester group compound with (B) the monovalent phenolic group compound in the presence of (E) the metal group reaction catalyst in the above solution to produce a mutually dissolving solution of the modified cyanate ester resin and the polyphenylene ether resin, and suspending the mutually dissolving resins by adding and agitating (G) the ketone group solvent.
The fourth object of the present invention can be achieved by the method for manufacturing prepreg for a laminated board, which Comprises impregnating the modified cyanate ester group resin varnish for a printed circuit board into a substrate, and subsequently drying the impregnated substrate at a temperature in the range of 80-200xc2x0 C., and the method for manufacturing a metal clad laminated board, which comprises piling up one or, plural prepregs for the laminated board, piling up one or plural metal foils at the upper and lower end planes of the pile, or either plane, and heating and pressurizing the pile to form the metal clad laminated board.
The fifth object of the present invention can be achieved by a method of manufacturing a multilayered circuit board comprising the steps of manufacturing a printed circuit board by patterning circuits onto the metal clad laminated board manufactured by the above method for obtaining a metal clad laminated board, wherein the metal is plated onto both side planes and/or one side plane, with a conventional process such as forming through-holes, metal plating, etching, and the like; laminating the prepreg, the film, or a combination of the prepreg and the film, onto the surface of the printed circuit board as an inner layer board, with a metal foil being further laminated thereon, by heating and pressurizing a pile of those materials; forming a window at a place on the metal foil, where a via-hole for connecting the inner layer circuit and the outer layer circuit must be formed, using a photosensitive resin or screen printing so that the metal foil at the place where the via-hole must be formed is exposed; forming via-holes through to the inner layer circuit by a laser beam drilling process using the metal foil as a mask after removing the metal foil exposed in the window by etching; connecting the inner layer circuit and the outer layer circuit by metal plating including the inner wall of the via-holes; and manufacturing circuits onto an outer surface of the metal foil to obtain the multilayered circuit board.
By repeating the above steps of the method of manufacturing a multilayered circuit board using the multilayered circuit board manufactured by the once-through operation of the above steps as the inner layer board, a multilayered circuit board having a large number of layers can be manufactured.
Another multilayered circuit board can be manufactured by using a film adhered with a metal foil instead of the prepreg and the film in the above steps, piling up the film adhered with the metal foil onto inner layer board so that the film plane is contacted with the inner layer board, and applying heating and pressurizing to laminate them.
The dielectric property of a polymer material is strongly affected by the polarization alignment of dipoles. Therefore, the dielectric constant can be decreased by decreasing the number of polar groups in the molecule, and the dissipation factor can be lowered by suppressing the mobility of the polar groups. Since a cyanate ester resin produces a symmetric and rigid triazine structure when being cured, though it has a strong polar cyanate group, a cyanate ester resin can provide a cured material having the lowest dielectric constant and dissipation factor among the thermosetting resins.
However, in the actual curing reaction, the cyanate groups in the cynate ester resin cannot all react to produce the triazine structure. The reaction system gradually loses its fluidity as the curing reaction progresses, and some cyanate groups remain in the system as non-reacted cyanate groups. As a result, only a cured resin having a dielectric constant and dissipation factor values higher than what the cured resin should essentially have was manufactured.
On the other hand, the resin composition in accordance with the present invention is aimed at decreasing the dielectric constant and the dissipation factor of the cured resin by adding an appropriate quantity of (B) monovalent phenolic group compound in order to convert the remaining non-reacted cyanate groups into imidocarbonate for decreasing the polarity of the cured resin. A suitable material used for this purpose is a chemical composition which is highly reactive with the cyanate group, and is single functional, relatively low in molecular weight, and mutually soluble with a cyanate ester resin (similar in molecular structure). The monovalent phenolic group compounds used in the resin composition of the present invention are specified by the above reason.
Conventionally, a phenolic compound such as nonylphenol and the like was used as an auxiliary catalyst for trimerizing reaction of cyanate ester (forming triazine rings) by adding approximately 1 to 2 parts by weight to 100 parts by weight of cyanate ester. However, since the amount added was actually a catalyst quantity, the effect of the phenolic compound to decrease the polarization by reacting with the cyanate group as above was not observed. According to the inventors"" study of the added quantity of the phenolic compounds, it was found that the dielectric constant and the dissipation factor of the cured material could be decreased by adding more of the phenolic compound than the conventional quantity, and a decrease in heat resistance due to increase in the added quantity of the phenolic compound could be suppressed by using a specific monovalent phenolic group compound. Therefore, according to the method of the present invention, it became possible to obtain a cured material having a dielectric constant and a dissipation factor lower than those of the conventional cured materials of a single cyanate ester resin and the conventional cured material of resin composed of epoxy resin, a multivalent phenol group (hydroxyl in one side was apt to remain as a non-reacted group, which deteriorated the dielectric property), bismaleimide, and the like.
Therefore, in accordance with the modified cyanate ester group curable resin composition of the present invention, the added quantity of the monovalent phenolic group compound is important. That is, the monovalent phenolic group compound cannot react with all the non-reacted remaining cyanate group to decrease the polarization when the quantity added is small, and the monovalent phenolic group compound itself remains as non-reactants to deteriorate the dielectric property of the cured material by the polarity of the hydroxyl of the monovalent phenolic group compound itself when the quantity added is more than necessary.
Further, in accordance with the modified cyanate ester group curable resin composition of the present invention, improving the dielectric property is intended by adding (C) the polyphenylene ether resin, which is a thermoplastic resin having a desirable dielectric property, to the modified cyanate ester resin. The cyanate ester resin and the polyphenylene ether resin are essentially not soluble with each other, and it is difficult to obtain a uniform resin. However, according to a technique found by the inventors, when (A) the cyanate ester group compound and (B) the monovalent phenolic group compound were reacted in a solvent solution of polyphenylene ether resin, a uniform resin solution could be manufactured by forming a so-called xe2x80x9csemi-IPN (interpenetrating polymer network) resinxe2x80x9d.
The flame retardant used in the resin composition of the present invention must be not reactive with the cyanate ester group compound so as not to interfere with the reaction between (A) the cyanate ester group compound and (B) the monovalent phenolic group compound. Such a flame retardant is an alicyclic flame retardant (aliphatic ring type flame retardant) which is a hydrocarbon group low polar composition and accordingly it hardly deteriorates the dielectric property of the cured material. Further, another kind of specified flame retardant is easily miscible-with cyanate ester resin cured material because the specified flame retardant has a triazine structure similar to the cyanate ester cured material, and the specified flame retardant can give a flame retardant effect to the cyanate ester cured material without deteriorating the heat resistance and the dielectric property.
(A) A modified cyanate ester group curable resin composition in accordance with the present invention comprises (A) a cyanate ester group compound expressed by the chemical formula (1), (B) a monovalent phenolic group compound expressed by the chemical formula (2) or an alkyl substituted phenolic compound expressed by the chemical formula (3), (C) a polyphenylene ether resin, (D) a flame retardant not reactive with the cyanate ester group compound and (E) a metal group reaction catalyst, as essential components.
In accordance with the present invention, (A) the cyanate ester group compound is a cyanate ester group compound having two cyanate groups in one molecule as expressed by the chemical formula (1). The chemical compounds expressed by the chemical formula (1) are, for example, bis(4-cyanato-phenyl)ethane; 2,2-bis(4-cyanato-phenyl)propane; bis(3,5-dimethyl-4-cyanato-phhenyl)methane; 2,2-bis(4-cyanato-phenyl)-1,1,1,3,3,3-hexafluoropropane; xcex1,xcex1xe2x80x2-bis(4-cyanato-phenyl)-m-diisopropylbenzen; a cyanate ester compound of phenol added dicyclopentadiene polymer; and the like. Among them, any one or a mixture of 2,2-bis(4-cyanato-phenyl) propane and bis(3,5-dimethyl-4-cyanato-phenyl) methane is preferable, because a balance between the dielectric property and the moldability of their cured material is particularly desirable. The (A) the cyanate ester group compounds can be used as a single kind or a mixture of two or more kinds.
In accordance with the present invention, (B) the monovalent phenolic group compound is a monovalent phenolic group compound expressed by the chemical formula (2) or an alkyl substituted phenolic compound expressed by the chemical formula (3), and a compound having desirable heat resistance is preferable. The chemical compound expressed by the chemical formula (2) is, for example, p-(xcex1-cumyl) phenol, and the chemical compounds expressed by the chemical formula (3) are, for instance, p-tert-butylphenol, p-tert-amylphenol, and p-tert-octylphenol. (B) the monovalent phenolic group compounds can be used by a single kind or a mixture of two or more kinds.
The added quantity of (B) the monovalent phenolic group compound in the present invention is desirably 4 to 30 parts by weight to 100 parts by weight of (A) the cyanate ester group compound, preferably 5 to 30 parts by weight, and more preferably 4 to 25 parts by weight. When the added quantity of (B) the monovalent phenolic group compound is not more than 4 parts by weight, a sufficient dielectric property cannot be attained, and the dissipation factor particularly in a high frequency band is generally not decreased sufficiently. On the other hand, when the added quantity of (B) the monovalent phenolic group compound exceeds 30 parts by weight, the dissipation factor becomes rather high. This is not preferable. Therefore, in order to obtain a cyanate ester group resin cured material having a low dissipation factor in the high frequency band provided by the present invention, an appropriate quantity of (B) the monovalent phenolic group compound must be added to (A) the cyanate ester group compound.
In accordance with the present invention, (A) the cyanate ester group compound and (B) the monovalent phenolic group compound are used as the modified cyanate ester resin which is manufactured by reacting them. That is, they are used as a pre-polymer of (A) the cyanate ester group compound and imide-carbonated modified resin formed by adding (B) the monovalent phenolic group compound to (A) the cyanate ester group compound.
When (A) the cyanate ester group compound is reacted with (B) the monovalent phenolic group compound, the modified cyanate ester resin can be manufactured by adding all the appropriate quantity of (B) the monovalent phenolic group compound to be reacted from the beginning of the reaction, or the modified cyanate ester resin can be manufactured by adding a part of the appropriate quantity of (B) the monovalent phenolic group compound at an initial stage of the reaction, and after cooling, adding the remaining amount of (B) the monovalent phenolic group compound at a B-stage reacting time or curing time.
In accordance with the present invention, (C) the polyphenylene ether resins are, for example, an alloyed polymer of poly(2,6-dimethyl-1,4-phenylene) ether, or poly(2,6-dimethyl-1,4-phenylene) ether, with polystyrene, an alloyed polymer of poly(2,6-dimethyl-1,4-phenylene) ether with styrene-butadiene copolymer, and the like. Particularly, the alloyed polymer of poly(2,6-dimethyl-1,4-phenylene) ether with polystyrene or styrene-butadiene copolymer and the alloyed polymer of poly(2,6-dimethyl-1,4-phenylene) ether with styrene-butadiene copolymer are desirable. A polymer containing the alloyed polymer of poly(2,6-dimethyl-1,4-phenylene) ether of more than 50% by weight is desirable, because the dielectric property of the cured material is desirable, and more than 65% by weight is particularly preferable.
The added quantity of (C) the polyphenylene ether resin in the present invention is desirably 5 to 300 parts by weight to 100 parts by weight of (A) the cyanate ester group compound, preferably 10 to 200 parts by weight, and more preferably 15 to 100 parts by weight. When the added quantity of (C) the polyphenylene ether resin is not more than 5 parts by weight, a sufficient dielectric property cannot attained. On the other hand, when the added quantity of (C) the polyphenylene ether resin exceeds 300 parts by weight, the moldability is deteriorated, because the molten viscosity becomes high and the fluidity becomes low, and the reactivity with (A) the cyanate ester group compound is also deteriorated.
In accordance with the present invention, (D) examples of the flame retardant not reactive with the cyanate ester group compounds are, for example, 1, 2-dibromo-4-(1, 2-dibromoethyl) cyclohexane, tetrabromocyclohexane, hexabromocyclododecane, polybromodiphenylether, polystyrene bromide, polycarbonate bromide, and triphenylcyanate bromide group flame retardants expressed by the chemical formula (4), and the like. Particularly, 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane; tetrabromocyclooctane; hexabromocyclododecane; 2,4,6-tris (tribromophenoxy)-1,3,5-triazine are desirable, because the manufactured cured materials have preferable dielectric properties. 
(where, respective of 1, m, and n is an integer of 1 to 5, and the integers can be the same or different from each other.)
The added quantity of (D) the flame retardant not reactive with the cyanate ester group compounds of the present invention is desirably 5 to 30 parts by weight to 100 parts by weight of the total of (A) the cyanate ester group compound, (B) the monovalent phenolic group compound and (C) the polyphenylene ether resin, and preferably 5 to 20 weight part, and further preferably 10 to 20 parts by weight. When the added quantity of (D) the flame retardant not reactive with the cyanate ester group compound is not more than 5 parts by weight, the flame retardant effect is insufficient. When the added quantity of (D) the flame retardant exceeds 30 parts by weight, the heat resistance of the resin is decreased.
In accordance with the present invention, (E) the metal group reaction catalyst accelerates the reaction between (A) the cyanate ester group compound and (B) the monovalent phenolic group compound, and is used as the reaction catalyst in manufacturing the modified cyanate ester group curable resin composition and as the curing accelerator in manufacturing the laminated board. The metal group reaction catalysts being used are metallic catalysts such as manganese, iron, cobalt, nickel, copper, and zinc. Practically, the catalysts are used as an organic acid metal salt compound such as 2-ethylhexanoic salts, naphthenic salts, and as an organic metal complex such as acetylacetone complex. The same kind of the metal group reaction catalyst can be solely used or two or more kinds of the metal group reaction catalysts can be respectively used as the reaction catalyst in manufacturing the modified cyanate ester group curable resin composition and as the curing accelerator in manufacturing the laminated board.
The added quantity of (E) the metal group reaction catalyst of the present invention is desirably 1 to 300 ppm to 1 (g) of (A) the cyanate ester group compound, preferably 1 to 200 ppm, and more preferably 2 to 150 ppm. When the added quantity of (E) the metal group reaction catalyst is not more than 1 ppm, the reactivity and the curability become insufficient. On the other hand, when the added quantity exceeds 300 ppm, the reaction becomes difficult to control and the moldability is deteriorated, because the curing speed becomes too fast to be controlled. The timing for adding (E) the metal group reaction catalyst of the present invention can be at the time of manufacturing the modified cyanate ester group curable resin composition when the necessary quantity of the metal group reaction catalyst as a reaction accelerator and a curing accelerator is added together at a time, or at the time of manufacturing the modified cyanate ester group curable resin composition when the quantity of the metal group reaction catalyst necessary for accelerating the denaturalizing reaction is added and then at the time after completion of the reaction when the remaining catalyst or the other metallic catalyst is added and mixed as a curing accelerator.
An inorganic filler and other additives other than the above-mentioned essential components can be added to the modified cyanate ester group curable resin composition of the present invention. The usable fillers are silica, alumina, aluminum hydride, calcium carbonate, clay, talk, silicon nitride, boron nitride, titanium oxide, barium titanate, lead titanate, strontium titanate and the like. In accordance with the present invention, the quantity to be added is preferably less than 250 parts by weight to 100 parts by weight of the total resin composition, in order to obtain a uniform distribution in the adhered quantity of the resin and a desirable appearance when the resin varnish of the present invention is impregnated into the supporting material such as glass cloth.
The modified cyanate ester group curable resin composition of the present invention is used in manufacturing flame retardant films, films, other varnishes, prepregs for laminated board, and metal clad laminated boards, for example, in a manner as described below. That is, initially, the prepreg is manufactured by dissolving or suspending the modified cyanate ester group curable resin composition of the present invention into a solvent to form a varnish, impregnating the varnish into a base material such as glass cloth, and then drying the impregnated base material. Next, the metal clad laminated board with metallic films on both side surfaces or with a metallic film on one side surface is manufactured by laminating one or an arbitrary number of sheets of the prepreg, laminating metallic films on both sides surfaces or on one side surface of the laminated prepreg; and then heating and pressurizing the laminated prepreg. The flame retardant films and films can be manufactured by semi-curing or curing the modified cyanate ester group curable resin composition of the present invention.
Practical examples of the solvents used for making the varnish of the modified cyanate ester group curable resin composition in accordance with the present invention are an aromatic hydrocarbon having a boiling point in the range of 70-170xc2x0 C. such as benzene, toluene, xylene and the like, a hydrocarbon halide such as trichloroethylene, chlorobenzene, and the like, an amide group such as N, N-dimethyl formaldehyde, N, N-dimethylacetoamide, and the like, and nitrogen group solvents such as N-methylpyrolidone and the like. Particularly, aromatic hydrocarbon such as benzene, toluene, xylene, and the like are desirable. These solvents can be used as solely one kind or by mixing two or more kinds. The added quantity of the aromatic hydrocarbon is desirably 150 to 500 parts by weight to 100 parts by weight of (C) the polyphenylene ether resin, preferably 150 to 400 parts by weight, and more preferably 150 to 300 parts by weight.
Ketones having a boiling point in the range of 50-170xc2x0 C. such as acetone, methylethylketone, methy-lisobutylketone, cyclohexanone, and the like have a low solubility to the modified cyanate ester group curable resin composition, but have such an advantage that when the ketones are used together with the above-mentioned solvents, a high density and low viscosity solution can be manufactured by forming a suspension of the resin composition in accordance with the present invention. From this viewpoint, the solvents used for making the varnish of the modified cyanate ester group curable resin composition in accordance with the present invention are desirably mixed solvents of an aromatic hydrocarbon such as benzene, toluene, xylene, and the like, with ketones such as acetone, methylethylketone, methy-lisobutylketone, cyclohexanone, and the like. The quantity of the ketone to be added is desirably 50 to 500 parts by weight to 100 parts by weight of the organic hydrocarbon group solvent, preferably 50 to 400 parts by weight, and more preferably 50 to 300 parts by weight.